Scientists May Be Missing Many Star Explosions

By SPACE.com Staff |
December 27, 2010 08:59am ET

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Artist's impression of the central system of the recurrent nova RS Ophiuchi early in its 2006 outburst. Hydrogen-rich material from the red giant falls onto the surface of its companion white dwarf, leading to a runaway thermonuclear explosion. Ejecta travelling at several thousand kilometers per second slams into the wind of the red giant, setting up shocks with temperatures more than 10 times higher than in the sun's core. RS Ophiuchi may end its life as a Type Ia supernova.

Credit: STFC/David Hardy.

Some of the
brightest stellar explosions in the galaxy may be flying under
astronomers' radar, a new study suggests.

Researchers using
observations from a sun-studying satellite detected four novas 
exploding
stars not quite as bright or dramatic as
supernovas. The scientists were able to follow the explosions in
intricate detail over time, including before the novas reached
maximum brightness.

While other
astronomers had discovered all four novas before, two of them escaped
detection until after they had reached peak luminosity, the study
revealed. This fact suggests that many other stellar explosions 
even some that are incredibly bright  may be occurring unnoticed,
researchers said. [Illustration
of nova]

"So far, this
research has shown that some novae become so bright that they could
have been easily detected with the naked eye by anyone looking in the
right direction at the right time, but are being missed, even in our
age of sophisticated professional observatories," study lead
author Rebekah Hounsell, a graduate student at Liverpool John Moores
University (LJMU) in England, said in a statement.

The new observations
are also allowing scientists to study nova explosions in
unprecedented detail, according to researchers.

"The data
have in fact opened a new chapter in our observations and
understanding of novae," Hounsell added.

A sun-monitoring
satellite

Hounsell and her
colleagues analyzed measurements from an instrument aboard the U.S.
Department of Defense's Coriolis satellite. The instrument, called
the Solar Mass Ejection Imager (SMEI), was designed to detect
disturbances in the
solar wind. SMEI maps out the entire sky
during its 102-minute orbit around the Earth.

The researchers
found that SMEI was also detecting star explosions, or novas. Novas
occur when small, extremely dense stars called white
dwarfs suck up gas from a nearby
companion star, igniting a runaway thermonuclear explosion.

Unlike supernovas,
novas do not result in the destruction of their stars. Stars can go
nova repeatedly.

SMEI detected four
novas, including one confirmed repeater called RS Ophiuchi, which is
found about 5,000 light-years away in the constellation Ophiuchus. RS
Ophiuchi may ultimately die in a supernova
exposion  one of the brightest, most
dramatic events in the universe  researchers said.

Ground-based
instruments missed
the peak flare-up of two of these four
novas, according to researchers. That suggests that space-based
instruments like SMEI might be needed to pick up many novas, after
which their progress can be tracked with telescopes on the ground,
researchers said.

"Two of the
novae observed by SMEI have confirmed that even the brightest novae
may be missed by conventional ground-based observing techniques,"
said co-author Mike Bode, also of LJMU.

The researchers
reported their results in a recent issue of the Astrophysical
Journal.

Getting to know
novas

The new observations
are giving astronomers key insights into novas' earlier days,
revealing a great deal about how they start and evolve, researchers
said.

"The SMEI's
very even cadences and uniformly exposed images allow us to sample
the sky every 102 minutes and trace the entire evolution of these
explosions as they brighten and dim," said co-author Bernard
Jackson of the University of California, San Diego.

The new observations
have revealed, for example, that three of the explosions faltered
significantly before regaining strength and proceeding. Such a
"pre-maximum halt" had been theorized before, but evidence
for its existence had been inconclusive, researchers said.

Since SMEI performs
a survey of the entire sky every 102 minutes, the instrument could
also help astronomers understand a wide variety of transient objects
and phenomena, according to the research team.

"[This] work
has shown how important all-sky surveys such as SMEI are and how
their data sets can potentially hold the key to a better
understanding of many variable objects," Bode said.